The present disclosure relates to the field of radiography and, in particular, relates to computer tomography scanners. Even more particularly, the present disclosure relates to a collimator and a collimator assembly for use with a computer tomography scanner.
In computed tomography, a patient to be examined is positioned in a scan circle of a computer tomography scanner. A shaped x-ray beam is then projected from an x-ray source through the scan circle and the patient, to an array of radiation detectors. By rotating the x-ray source and the collimator relative to the patient (about a z-axis of the scanner), radiation is projected through an imaged portion of the patient to the detectors from a multiplicity of directions. From data provided by the detectors, an image of the scanned portion of the patient is constructed.
Within the x-ray source, an electron beam strikes a focal spot point or line on an anode, and x-rays are generated at the focal spot and emitted along diverging linear paths in an x-ray beam. A collimator is employed for shaping a cross-section of the x-ray beam, and for directing the shaped beam through the patient and toward the detector array.
Conventional collimators generally comprise a flat plate with a rectangular slit of uniform width for producing a rectangular beam cross-section, as desired with systems employing a rectangular detector array. The conventional collimator design is problematic, however, since the actual cross-sectional shape of the beam produced by the collimator is not precisely rectangular but is instead wider at its center than at its ends, i.e., convex. The convex beam cross-section may extend beyond a desired row of detectors and irradiate adjacent rows of detectors. In addition, the convex beam cross-section may subject a patient to a dose of x-rays in excess of those required for the scan.
Conventional collimators produce such convex beam cross-sections because of the variation in distance between the focal spot of the x-ray source and different portions of the flat slit of the collimator through which the beam passes. An example of a convex beam cross-section produced by such conventional collimators is illustrated in FIGS. 1 and 2.
In a conventional computed tomography scanner 1, as represented in FIGS. 1 and 2, an x-ray source 2 projects a beam 4 from a focal spot 3, through a slit 12 in a collimator 10. The resulting cross-section 6 of the beam 4, as incident on a detector array 8 for example, is wider slightly in its center portion 7a, as compared to end portions 7b of the beam cross-section 6.
More particularly, the center portion 7a of the beam cross-section 6 has a width w1 that is wider than a width w2 of each of the end portions 7b. This results because a distance d1 between the focal spot 3 and a center portion 14a of the slit 12 is greater than a distance d2 between the focal spot 6 and end portions 14b of the slit 12. As shown in FIG. 2, if the widths w2 of the end portions 7b of the beam cross-section 6 are matched to the widths W of end detectors 9b of the detector array 8, then the width w1 of the center portion 7a of the beam cross-section 6 extends beyond the width W of centrally located detectors 9a of the detector array 8. A patient being scanned, therefore, may be subject to an unnecessary radiation dose since the portion of the beam cross-section extending beyond the detectors is unused.
Another problem associated with conventional computer tomography scanners arises due to component movement, or drifting, that occurs during operation of the scanners. Control of these movements can be critical since accurate image generation through computer tomography scanning assumes that the components of the system, especially the focal spot, collimator and detectors, always remain perfectly aligned relative to one another during a scan, and from scan to scan. Consequently, any movement of the various tomography components during a scan can cause major inaccuracies in reconstructed images.
One particular cause of unwanted movement is the beam source itself. For example, as the anode of the beam source heats up during operation, thermal expansion causes the focal spot to shift, thus causing the resulting x-ray beam to shift with respect to the collimator. Typically, the focal spot will drift in a direction parallel to the z-axis of the scanner. The focal spot shifting can detract from the integrity of the image data and can cause major inaccuracies in the reconstructed image.
What is desired, therefore, is a collimator that produces a beam cross-section having a uniform width. What is also desired is a collimator assembly providing a plurality of collimator slits of varied widths for selective alignment between a focal point and a detector array of a computer tomography scanner.
What is additionally needed and desired is a collimator assembly that compensates for shifting of a focal point of a computer tomography scanner during a scanning procedure, to ensure proper alignment of a collimator of the assembly with the focal spot.
The present disclosure is directed to a collimator and collimator assembly that address and overcome the limitations of conventional collimators and computer tomography scanners. In particular, the present disclosure provides a collimator including a plurality of slits that each have a uniform width and are each curved about a common axis of curvature for producing a beam cross-section of a substantially uniform width. In addition, the slit widths are varied from one another for producing beam cross-sections of varied widths. Furthermore, the collimator is shaped so that the slits can be sequentially aligned with a focal point of a computer tomography scanner by rotating the collimator about a rotation axis normal to the axis of curvature.
The present disclosure also provides an assembly for selecting one of the slits of the collimator. The assembly includes a selection motor having a rotatable shaft, and a gear mechanism coupling the motor shaft to the collimator for rotating the collimator about its rotation axis to select a slit. According to one aspect, a resilient material is seated in a circumferential groove of at least one gear of the gear mechanism for absorbing shock. According to another aspect, an index pin is provided for receipt in an index aperture of the gear mechanism for fine tuning and locking the rotated position of the collimator.
The present disclosure additionally provides an assembly that realigns the collimator with a shifting focal point of a computer tomography scanner during a scanning procedure, to ensure proper alignment of the collimator and the focal point. The assembly includes an alignment motor having a rotatable shaft, a cam fixed to the motor shaft for rotation therewith, and a follower rotatably and slidingly received on the motor shaft and operatively contacting the cam for axial movement of the follower along the shaft upon rotation of the cam. The collimator is operatively coupled to the follower for movement of the collimator in a direction parallel to the shaft of the motor upon movement of the follower. Preferably, the alignment motor is oriented such that the collimator moves parallel to a z-axis of a scanner. According to one aspect, the assembly includes a spring biasing the collimator toward the alignment motor.